In prior U.S. Pat. No. 3,866,845, which is assigned to the assignee of the subject application, a thermoformed closure for a container is disclosed in which the closure material is of two sheets of material which are bonded together. In a preferred embodiment of the invention, one of the sheets of material is a relatively hard plastic, for example polystyrene, polyethylene, etc., while the other sheet is of a softer material, for example of a rubber-like plastic material such as that sold under the trademark KRATON. In the closure of that patent, the two sheets of material are bonded together, for example by laminating, before the laminate is placed in a thermoforming mold. The thermoforming operation is carried out in a manner such that the sheet of softer material is on the inside when the closure is formed. The purpose of the closure of the aforesaid patent is to eliminate the need for a separate liner, which is usually glued or force fit into the closure, and also to eliminate the separate lining operation.
The thermoplastic rubber material used for the preferred embodiment of closure disclosed in the prior United States patent, although effective in providing a seal, had certain disadvantages. One disadvantage of the thermoplastic rubber material is its relatively high cost. Of course, less expensive plastic materials which are softer could be used. Another disadvantage was the high coefficient of friction associated with a thermoplastic rubber material. This causes some difficulty in tightening the closure onto the container. This can be overcome by special treatment of the liner material to reduce the coefficient of friction.
The present invention relates to an improved closure of the foregoing type in which a composite of two materials is used for a thermoformed closure. The composite includes an outer layer, or skin, to which is bonded an inner layer used for the closure liner. The two materials of the composite are plastic and can be from the same family of materials or from different families.
In the preferred embodiment of the invention, a plastic material is used which has an outer layer which is relatively hard. To this is bonded an inner layer of a cellular expanded foam plastic of either the same or different material than the outer layer. Since the cellular expanded foam plastic material has a low coefficient of friction, less torque is required to tighten the closure than was required in the previous invention. Furthermore, since the expanded foam is somewhat compressible, the inner liner conforms to the shape of the threads and to the shape of the sealing lip of the container, thereby creating a tight seal.
Another aspect of the invention is the use of a composite plastic material of a cellular foam material with an integral skin. Here, the integral skin forms the outer shell material, thus obviating the need to bond separate sheets of plastic material together to form a laminate. That is, the composite skin and foam are formed from a single sheet of material, for example, by expanding a part of the sheet material into the foam.
It is therefore an object of the present invention to provide a novel closure thermoformed of a composite of materials, one of which is an expanded plastic foam.
A further object is to provide a thermoformed closure made from a composite of materials, one of which is an expanded cellular plastic foam, in which the foam serves as a liner.
Another object is to provide a thermoformed closure using an expanded plastic foam material with an integral outer skin.